[unreadable] To-date most real time imaging procedures have been used to detect cancer cells in vivo. The application of these methods may not be appropriate for stem cells for several reasons. First, the tracking systems include the expression in the cells to be tracked of a foreign protein that has the potential of altering stem cell behavior such as proliferation, differentiation, and migration. Second, the production of the foreign protein, often to reach many fold over the levels of other proteins in the cell, requires that the cell devote a significant portion of its energy to its synthesis. Although this is not likely to be a problem for cancer cells which generally express very high levels of hexokinase and glucose transporters for making ATP by glycolysis, stem cells are less robust and could be debilitated. For effective tissue repair, stem cells must migrate and proliferate in regions of previously damaged tissues that are often hypoxic and nutrient depleted. [unreadable] [unreadable] The additional need to synthesize a foreign protein from a constitutive promoter that cannot readily be down regulated will place an energy burden on the cells when they need to cope with an inadequate environment and may decrease the number of cells that can successfully populate the damaged tissue. Consequently, for marking stem cells, it would be of great advantage to have a system that does not place a large energy burden on the cells that are being imaged. [unreadable] [unreadable] The ultimate goal of the research proposed here is to develop an innocuous means of marking stem cells so that they can be tracked in vivo and in real time without their potential for homing, proliferation and differentiation being altered by the marking system. The current proposal describes a novel nucleic acid structure that will be expressed in cells to mark them so they can be tracked in vivo by available imaging technology. We call the markers "Intracellular MultiAptamer Genetic Tags" (IMAGEtags). The basic innovation in the proposed IMAGEtag design is the expression in living cells of aptamers that can be used to track the cells in vivo by a noninvasive procedure. Cells that express IMAGEtags will be detected by virtue of the concentrated ligands bound to the aptamers. (End of Abstract) [unreadable] [unreadable]